In October the team that invented the blue LED – Isamu Akasaki, Hiroshi Amano and Shuji Nakamura – won the Nobel physics prize for their work. The prize is usually won by quantum physicists or cosmologists, so people in the lighting industry are understandably feeling rather chipper – because for once, some of our lot are getting a share of the glory.
So now is the perfect time to look at how white LEDs work, and examine a whole new direction for the technology.
Basically, a blue LED works by applying an electric field to a gallium nitride (GaN) crystal. When this happens, an electron breaks free from the atom, and then recombines, releasing a photon of blue light every time. Cover the blue LED with a phosphor and some of this blue light is converted to white. GaN crystals are made by ‘growing’ them in a film on top of another crystalline material, so that the crystal lattices match up. But when Nakamura and his team were developing this blue LEDs back in the 1990s, growing GaN crystals from scratch was very difficult, involving very high temperatures and pressures. It was easier and more economical to grow the crystals on sapphire substrates, but the downside of this approach is that the crystal lattices don’t align so well, which means less light output and shorter lifetimes.
Over the past couple of years I’ve heard about new LEDs based on GaN crystals grown on GaN substrates. Having learned a thing or two about crystal growth when I worked in device physics, I was sceptical, given the difficulties involved and the cost of production.
Well, I’m happy to say that Soraa, founded by Shuji Nakamura, has proved me wrong. I don’t know how they’ve done it (which I suppose is why it’s Nakamura that got the Nobel Prize and not me) but they’ve created a GaN-on-GaN LED that is affordable and competes on quality of light and beam control with the best units on the market.
The product we’ll look at is Soraa’s constant-current MR16 LED 3000K unit. One of the advantages of using GaN-on-GaN crystals is the high current density, so you can use much smaller LED chips. This, combined with Soraa’s point source optics, means you can have tighter control of the beam angle. In the example I tested, we saw a beam angle of 10 degrees and a peak luminous intensity of 7,670Cd.
Until recently I’d always been slightly sceptical of peak luminous intensity as a selling point – some unscrupulous people have marketed ‘peak candela’ to cover the fact that in other respects, such as total output, their luminaires don’t perform so well. Narrowing the beam and increasing the lumens per solid angle doesn’t necessarily make a good luminaire. However, when we’re talking about a display lighting unit like this 10-degree MR16 replacement, lighting designers like to have a bit of ‘sparkle’ and tight beam control gives them that. Soraa also has a range of snap-on magnetic optical accessories for the lamp, such as beam shapers and colour filters.
I was impressed by the measured high CRI value of 93 (Soraa claims 95 for CRI and R9, which puts our results well within experimental tolerances), but what especially interested me was how Soraa achieved this. One of the consequences of using GaN crystals is that the blue peak shifts from 440nm to about 410nm. This, combined with the mix of three phosphors on the LED, gives us a marvellous broadband spectrum, with high colour-rendering indices right across the board (see table, below left).
Whiter than white
This peak shift also creates a higher perception of ‘whiteness’. Many white materials contain optical brightening agents (OBAs) that compensate for the yellowing of the material over time. OBAs absorb light at short wavelengths and radiate light at longer wavelengths. This increases the perceived brilliance and whiteness of a material through a combination of luminescence and its resultant chromatic shift towards blue. The combination of the 410nm GaN emission and Soraa’s triphosphor technology exploits this effect, with positive consequences for the brightness of the material that is being illuminated.
The high power factor of 0.937 and low input power of 11W also contribute to the success of this MR16 replacement. Soraa engineers achieved this by using an externally mounted Truelux driver that has been specially developed for Soraa luminaires. This ensures a linear output over the full voltage range with effective, flicker-free dimming.
The lamp squeezes out a lot more light than the version Lux first tested back in 2013, for less power. A total luminous flux output of 510 lumens gives us an efficacy of 46.4 lm/W. I’ve seen some LED-based MR16 replacements with higher efficacy than this, but not with such high CRI and power factor.
Dr Gareth John is technical director of Photometric Testing, an independent lighting test laboratory that specialises in the photometric assessment of LEDs, luminaires, lamps and displays.